1. Field of the Invention
The present invention relates to a lens for reading out color images or color originals; and, in particular, to a color image readout lens, having a brightness with an effective F number of about 8 to 11, adapted to read out images on negative or positive films over a range of reducing and enlarging magnifications including an actual size.
2. Description of the Prior Art
Recently, developing machines known as mini lab or digital lab has come into widespread use. In these developing machines, images on a film are not directly printed onto paper but once captured on a solid-state imaging device such as CCD by use of a lens, so that they can be subjected to various processes, and then are printed onto paper by use of laser or the like. In such a developing machine, its image readout optical system for forming images onto the solid-state imaging device is required to have a high resolution, since its corresponding light-receiving devices have a high density.
In the case where a color original is read out by a three-line CCD, for example, it is necessary that an achromatizing process be favorably carried out for each color of red (R), green (G), and blue (B) such that their respective imaging positions coincide with each other on the light-receiving surface of sensor, and it is desirable that the individual colors keep substantially the same level of performances on the sensor. Specifically, it is necessary that the axial chromatic aberration and chromatic aberration in magnification of each color of R, G, and B be made smaller, and their imaging points from the image center to peripheries be matched, so that each color forms an image with a high contrast at the same position. If the amount of correction of axial chromatic aberration is small, then the respective image point positions of individual colors may shift from each other. Consequently, even when the individual colors have high imaging performances by themselves, it is hard to reproduce their high performances at the same imaging position. Also, in order to correct them, it is necessary to use such means as a mechanism for separately carrying out focusing for each color, the shifting of the imaging surface according to the amount of chromatic aberration generated, and the like.
On the other hand, letting f be the focal length of a lens, and .beta.(.beta.&lt;0) be the imaging magnification thereof, the generated amount of axial chromatic aberration (.DELTA.S) has a relationship of .DELTA.S .alpha.(1-.beta.).sup.2.multidot.f, thus increasing as the focal length is longer or the absolute value of magnification .vertline..beta..vertline. is greater. This property is remarkable in lenses using no materials with anomalous dispersion, whereby axial colors may not be corrected sufficiently. In enlargement ranges, in particular, higher resolution and higher contrast of R, G, and B may not be realized in the same image surface. For correcting this large axial chromatic aberration, it is therefore effective to use a material having a large anomalous dispersion. The correcting effect becomes higher as the material has a greater anomalous dispersion, and the correcting force becomes stronger as its refracting power is stronger. When the refracting power is too strong, however, then various aberrations may occur greatly, thereby deteriorating image quality. As a lens system, it is desirable that the value of the product obtained by multiplying the refracting power .phi..sub.i (=1/f.sub.i) of an element using a material with an anomalous dispersion by the anomalous dispersion .delta..theta..sub.i be made greater, so as to further correct other aberrations favorably.
As a conventional technique attempting achromatization, one disclosed in Japanese Unexamined Patent Publication No. 4-311912 or the like has been known.
However, the technique disclosed in the above-mentioned publication is used at a magnification of about -0.2205X, and thus aims at forming images under reduction, and does not aim at correcting a large axial chromatic aberration when forming images under magnification. Therefore, for example, when used within the range from -0.6X to -1.7X including an actual size therein, favorable performances are hard to obtain on the enlarging magnification side in particular.
Japanese Unexamined Patent Publication No. 10-325921, on the other hand, discloses a lens system in which a material with an anomalous dispersion is used for a lens, whereby chromatic aberration is corrected upon forming images under magnification. However, due to restrictions concerning correction of other aberrations, the material with an anomalous dispersion cannot attain a predetermined refracting power or greater. Therefore, it is hard to correct axial chromatic aberration sufficiently, and the correction becomes more difficult when the lens system is used at a greater enlarging magnification. Also, fluctuations in aberration upon power variations are not taken into consideration, whereby favorable performances are hard to obtain over a range from reduced imaging to enlarged imaging.